1. Field of the Invention
The present invention relates to magneto-resistive elements that are widely used, for example, in magnetic random access memory (MRAM) used in data communication terminals, for example, and to manufacturing methods for the same.
2. Description of the Related Art
It is known that when a current flows through a multilayer film including ferromagnetic material/intermediate layer/ferromagnetic material in a direction traverse to the intermediate layer, a magneto-resistive effect occurs due to the spin tunneling effect if the intermediate layer is a tunneling insulating layer, and a magneto-resistive effect occurs due to the CPP (current perpendicular to the plane)-GMR effect if the intermediate layer is a conductive metal, such as Cu. Both magneto-resistive effects depend on the size of the angle between the magnetizations of the magnetic materials sandwiching the intermediate layers (magnetization displacement angle). In the former, the magneto-resistive effect occurs due to changes of the transition probability of tunneling electrons flowing through the two magnetic layers depending on the magnetization displacement angle, and in the latter the magneto-resistive effect occurs due to changes in the spin-dependent scattering.
When such a TMR element is used for a magnetic head or an MRAM device, one of the two magnetic layers sandwiching the intermediate layer can serve as a pinned magnetic layer, in which magnetization rotations with respect to an external field are difficult, by layering an antiferromagnetic material of FeMn or IrMn onto it, whereas the other layer serves as a free magnetic layer, in which magnetization rotations with respect to an external field are easy (spin-valve element).
When applying these vertical current-type resistive elements for example to a magnetic head or memory elements of an MRAM, for example in a reproduction element for tape media, then the area of the intermediate layer through which current flows should be not larger than several 1000 xcexcm2, in order to achieve the demanded high recording densities or high installation densities. Especially in HDDs and MRAMs or the like, an element area of not more than several xcexcm2 is desired. If the element area is large, magnetic domains form relatively easily in the free magnetic layer Therefore, there are the problems of Barkhausen noise due to magnetic wall transitions when used as a reproduction element, and instabilities of the switching magnetization when used for the memory operation of MRAMs. On the other hand, in a region, in which the film thickness of the free magnetic layer with respect to the element area cannot be ignored, the demagnetizing field due to shape anisotropies becomes large, so that especially when used as a reproduction head, the decrease of the reproduction sensitivity brought about by an increase of the coercivity becomes a problem. When used as an MRAM, the increase of the reversal magnetic field becomes a problem.
In order to suppress the demagnetizing field, the film thickness of the free magnetic layer can be made thinner. However, at submicron dimensions, the film thickness of the magnetic layer necessary to suppress the demagnetizing field becomes less than 1 nm, which is below the physical film thickness limit of magnetic films.
Using the TMR elements for an MRAM, a thermal process at about 400xc2x0 C. is performed in a semiconductor process of hydrogen sintering or a passivation process. However, it has been reported that in conventional pinned layers, in which IrMn or FeMn is arranged in contact with a magnetic layer, the MR is decreased by the decrease of the spin polarizability of the magnetic layer due to diffusion of Mn at temperatures of about 300xc2x0 C. or above, and the decrease of the pinning magnetic field due to the dilution of the composition of the antiferromagnetic material (see S. Cardoso et.al., J. Appl.Phys. 87, 6058(2000)).
In previously proposed methods for reading non-volatile MRAM elements, the read-out is difficult when there are large variations in element resistance or in the resistance of switching element and electrode, because what is read out is the change of magnetic resistance of the magneto-resistive element with respect to the total resistance of magneto-resistive elements connected in series to a switching element and an electrode. In order to improve the SIN, a method of reading an element with the voltage between that element and a reference element has been proposed, but in that case, the higher integration of the elements becomes a problem, because the reference element is necessary (see p. 37, Proceedings of 112th Study Group of the Magnetics Society of Japan).
According to a first aspect of the present invention, a vertical current-type magneto-resistive element includes an intermediate layer and a pair of magnetic layers sandwiching the intermediate layer, wherein one of the magnetic layers is a free magnetic layer in which magnetization rotation with respect to an external magnetic field is easier than in the other magnetic layer, wherein the free magnetic layer is a multilayer film including at least one non-magnetic layer and magnetic layers sandwiching the non-magnetic layer, and wherein an element area through which current flows is not larger than 1000 xcexcm2, preferably not larger than 10 xcexcm2, more preferably not larger than 1 xcexcm2, most preferably not larger than 0.1 xcexcm2. The element area is defined by the area of the intermediate layer through which the current flows perpendicular to the film plane. Providing the free magnetic layer as a multilayer structure of magnetic and non-magnetic layers suppresses the demagnetizing field, which increases as the element area becomes smaller. Here, the magnetic and non-magnetic layers can be single layers or multilayers. It is preferable that the free magnetic layer performs magnetization rotation at an external magnetic field causing magnetization rotation that is at least 50 Oe (ca. 4 kA/m) smaller than that required for magnetization rotation of the other magnetic layers (usually, the pinned magnetic layer). Especially when the element is used for a memory, it is preferable that magnetization rotation at a value of 10 to 500 Oe is possible.
It is preferable that, in particular near 0.5 nm of the interface with the intermediate layer, the magnetic layers are made of a ferromagnetic or ferrimagnetic material including at least 50 wt % of (i) a Co-based amorphous material such as CoNbZr, CoTaZr, CoFeB, CoTi, CoZr, CoNb, CoMoBZr, CoVZr, CoMoSiZr, CoMoZr, CoMoVZr or CoMnB, (ii) an Fe-based microcrystal material, such as FeSiNb or Fe(Si,Al,Ta,Nb,Ti)N, (iii) a magnetic material containing at least 50 wt % of a ferromagnetic metal element selected from Fe, Co and Ni, for example ferromagnetic or dilute magnetic materials like FeCo alloy, NiFe alloy, NiFeCo alloy, FeCr, FeSiAl, FeSi, FeAl, FeCoSi, FeCoAl, FeCoSiAl, FeCoTi, Fe(Ni)Co)Pt, Fe(Ni)(Co)Pd, Fe(Ni)(Co)Rh, Fe(Ni)(Co)Ir or Fe(Ni)(Co)Ru, (iv) a nitride, such as FeN, FeTiN, FeAlN, FeSiN, FeTaN, FeCoN, FeCoTiN, FeCoAlN, FeCoSiN, FeCoTaN, (v) Fe3O4, (vi) a half metal, such as XMnSb (wherein X is at least one selected from Ni, Cu and Pt), LaSrMnO, LaCaSrMnO or CrO2, (vii), a spinel oxide such as a perovskite oxide, MnZn ferrite or NiZn ferrite, or (viii) a garnet oxide. In this specification, elements or layers in parentheses are optional ones.
It is preferable that the area of the free magnetic layer is wider than the element area. If the area of the free magnetic layer is substantially the same as the element area, then the MR decreases due to the influence of disturbances of the domain structure that occur at the edge of the free magnetic layer. When the area of the free magnetic layer is larger than the element area, and when the free magnetic layer is formed to cover the element area sufficiently, then the edges of the free magnetic layer are separated from the element area, so that the magnetization direction inside the free magnetic layer that contributes to the magnetic resistance can be kept uniform.
It is preferable that the magneto-resistive element includes a non-magnetic layer with a thickness d in the range of 2.6 nmxe2x89xa6d less than 10 nm, because that facilitates magnetization rotation of the free magnetic layer. It seems that the demagnetizing field energy is reduced by the magnetostatic coupling of the magnetic poles occurring due to the shape anisotropy between the magnetic layers located on both sides of the non-magnetic layer. By providing the free magnetic layer with the above-described configuration, the magnetic domains are simplified, and a high MR can be attained. If xe2x80x9cdxe2x80x9d is 10 nm or more, then the magnetostatic coupling between the magnetic layers becomes weak and the coercivity increases. If xe2x80x9cdxe2x80x9d is less than 2.6 nm, then the exchange coupling becomes dominant. In that case, it is preferable that the thickness of the magnetic layers is at least 1 nm and at most 100 nm, in which range suitable magnetostatic coupling is attained. The non-magnetic material can be any non-magnetic metal, oxide, nitride or carbide, such as Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W Al, SiO2, SiC, Si3N4, Al2O3, AlN, Cr2O3, Cr2N, TiO, TiN, TiC, HfO2, HfN, HfC, Ta2O5, TaN, TaC, BN or B4C, and preferably it is a material at which mutual diffusion with the magnetic layer does not occur easily when thermally processing at 200xc2x0 C. to 400xc2x0 C.
It is preferable that when the magnetic layers constituting the free magnetic layer are coupled by magnetostatic coupling, taking the magnetic layers that are arranged at positions m (m=1, 2, . . . ) from the intermediate layer as magnetic layers m, and taking the product Mmxc3x97dm of average saturation magnetization Mm of the magnetic layers m and their average layer thickness dm, the sum of the products Mmxc3x97dm for odd m is substantially equal to the sum of the products Mmxc3x97dm for even m. This is, because by stopping magnetic field leakages from the magnetic layer and simplifying the magnetic domains, a magneto-resistive element with improved magnetization responsiveness with respect to an external magnetic field and with a higher MR can be attained. Here, xe2x80x9csubstantially equalxe2x80x9d means that differences of up to xc2x110% can be tolerated.
It should be noted that throughout this specification, xe2x80x9csaturation magnetizationxe2x80x9d means the value of the magnetization that can be effectively attained by the magnetic layers constituting the free magnetic layer when applying an external magnetic field of a size as under actual usage conditions of the element. That is to say, it is different from the saturation magnetization as determined by the material composition. This is because, especially in magnetic layers of about several nm thickness per layer, the domain forms that can be obtained depend on the substantial film density and film structure, which change with the type of primer layer and the growth process of the magnetic layer, as well as the film thickness.
When the magnetic layers constituting the free magnetic layer are coupled by magnetostatic coupling, the sum of the products Mmxc3x97dm for odd m may be different from the sum of the products Mmxc3x97dm for even m. When they are substantially the same, then the leakage flux between the magnetic layers constituting the free magnetic layer is mainly closed by magnetostatic coupling. Consequently, even when processing so as to provide the free magnetic layers with shape anisotropies, a bistable magnetization state, which usually can be observed in single-layer magnetic films, is difficult to attain. Therefore, the element becomes difficult to apply to devices in which the free magnetic layer serves as the memory. However, if the sum of the products Mmxc3x97dm for odd m is different from the sum of the products Mmxc3x97dm for even m, that is, if it is not substantially the same, then a magneto-resistive element can be attained, in which magnetization reversals are easy and a bistable magnetization state can be preserved.
It is preferable that the magneto-resistive element includes a non-magnetic layer with a thickness d in the range of 0.3 nm less than d less than 2.6 nm. The magnetization rotation of the free magnetic layer becomes easier when the thickness of the non-magnetic layer is in that range. This seems to be because the demagnetizing field energy is reduced by antiferromagnetic coupling between the magnetic layers adjacent to the non-magnetic layer. When the free magnetic layer has the above-described structure, the domains are simplified, and a high MR can be attained. When xe2x80x9cdxe2x80x9d is 0.3 or less, then the thermal stability deteriorates. When xe2x80x9cdxe2x80x9d is 2.6 nm or more, the magnetostatic coupling becomes dominant. Moreover, for the coupling brought about by a thermal process with at least 260xc2x0 C., xe2x80x9cdxe2x80x9d is preferably in the range of 0.8 nm to 2.6 nm. In that case, it is preferable that the thickness of the magnetic layers is at least 0.5 nm and at most 100 nm, in which range suitable antiferromagnetic coupling is attained.
The non-magnetic material can be a conductive metal or metal compound, and in particular, Cu, Ag, Au, Ru, Rh, Ir, Re and Os are preferable. Also preferable are alloys of these metals and alloys or compounds containing at least 50 wt % of these metal elements. These alloys and compounds have excellent thermal stability, and the element resistance is increased in particular when used for CPP-GMR.
When the magnetic layers constituting the free magnetic layer are coupled by antiferromagnetic coupling, it is preferable that taking the magnetic layers arranged at positions m (m=1, 2, . . . ) from the intermediate layer as magnetic layers m, and taking the product Mmxc3x97dm of average saturation magnetization Mm of the magnetic layers m and their average layer thickness dm, the sum of the products Mmxc3x97dm for odd m is different from the sum of the products Mmxc3x97dm for even m. If the product Mmxc3x97dm of the layers for odd m is the same as that of the layers for even m, then magnetization rotation is difficult when the external magnetic field is weak, because the magnetic layers are coupled by antiferromagnetic coupling. By making the extent to which the demagnetizing field increases different (for example, 0.5 to 2 Txe2x80xa2nm), magnetization rotations in response to an external magnetic field become even easier, and a magneto-resistive element with a low reversal magnetic field and high MR can be attained.
It is preferable that in the above-described element, the free magnetic layer includes a first magnetic layer, a non-magnetic layer and a second magnetic layer, layered in that order from the intermediate layer, and when an average film thickness of the first magnetic layer is d1, its average saturation magnetization is M1, an average film thickness of the second magnetic layer is d2, and its average saturation magnetization is M2, then
1.1 less than (M1xc3x97d1+M2xc3x97d2)/S less than 20; 
(wherein S is the absolute value of M1xc3x97d1xe2x88x92M2xc3x97d2); and (i) taking the effective film thickness d11 of the first magnetic layer as
xe2x80x83d11=(M1xc3x97d1xe2x88x92M2xc3x97d2)/M1
when M1xc3x97d1xe2x88x92M2xc3x97d2 greater than 0, and 
(ii) taking the effective film thickness d22 of the second magnetic layer as
d22=(M2xc3x97d2xe2x88x92M1xc3x97d1)/M2 
when M1xc3x97d1xe2x88x92M2xc3x97d2 less than 0, 
and taking as Nm the demagnetizing factor in the free magnetic layer surface in a direction of an applied external magnetic field, determined from the effective film thickness d11 or d22 and the free magnetic layer surface shape, then Nm less than 0.02. When the value of (M1xc3x97d1+M2xc3x97d2)/S is 1.1 or less, then the effect of simplifying the magnetic domains becomes weak, and when it is 20 or greater, then there is a considerable increase in the coercivity. Moreover, if the demagnetizing factor Nm of an applied magnetic field direction, determined from the surface shape of the free magnetic layer (for example, circular or rectangular) and the effective film thickness d11 or d22, is 0.02 or greater, then there are disturbances in the magnetic domain shape, and there is an increase in the energy necessary for magnetization rotation.
It is preferable that, when M2xc3x97d2 greater than M1xc3x97d1, the magnetic material constituting the second magnetic layer is a soft magnetic material or a hard magnetic material, and the magnetic material constituting the first magnetic layer is a high spin polarization material at least at an interface with the intermediate layer. If the product M2xc3x97d2 of the soft magnetic material is larger than the product M1xc3x97d1 of the high spin polarization material, then the magnetization rotation in response to an external magnetic field is easy, and a magneto-resistive element with few magnetic domain disturbances and a high MR can be attained. Such a magneto-resistive element with easy magnetization rotation can be used as a magnetic sensor, but if the free magnetic layer is used for a memory element, then it can be used as a stable magnetic memory by providing a suitable shape anisotropy in the film plane of the free magnetic layer. On the other hand, if the product M2xc3x97d2 of the hard magnetic material is larger than the product M1xc3x97d1 of the high spin polarization material, then the magnetization rotation in response to an external magnetic field is difficult, and a magneto-resistive element with few magnetic domain disturbances and a high MR can be attained. Using a hard magnetic material, a stable magnetic memory can be attained, even with small shape anisotropies. Here, it is preferable to use a material with a spin polarizability of at least 0.45, more preferably at least 0.5, as the high spin polarization material.
It is preferable to use, for example, CoPt, FePt, CoCrPt, CoTaPt, FeTaPt or FeCrPt as the hard magnetic material.
For the soft magnetic material, it is preferable to use an alloy such as Ni81Fel19, FeSiAl, FeSi or Fe90Co10, a Co-based amorphous material such as CoNbZr, CoTaZr, CoFeB, CoTi, CoZr, CoNb, CoMoBZr, CoVZr, CoMoSiZr, CoMoZr, CoMoVZr or CoMnB, an Fe-based microcrystal material such as FeSiNb or Fe(Si, Al, Ta, Nb, Ti)N, or an oxide material such as MnZn ferrite or NiZn ferrite.
In the high spin polarization material, it is preferable to include at least 50 wt % of (i) a metal ferromagnetic material of which at least 50 wt % is made of a ferromagnetic metal element selected from Fe, Co and Ni, for example a FeCo alloy with the composition FexCo(100-x)(15xe2x89xa6Xxe2x89xa6100), NiFe alloy with the composition NixFe(100-x)(40xe2x89xa6Xxe2x89xa670), NiFeCo alloy, ferromagnetic or dilute magnetic alloys, such as FeCr, FeSiAl, FeSi, FeAl, FeCoSi, FeCoAl, FeCoSiAl, FeCoTi, Fe(Ni)(Co)Pt, Fe(Ni)(Co)Pd, Fe(Ni)(Co)Rh, Fe(Ni)(Co)Ir and Fe(Ni)(Co)Ru, (ii) a nitride such as FeN, FeTiN, FeAlN, FeSiN, FeTaN, FeCoN, FeCoTiN, FeCoAlN, FeCoSiN, FeCoTaN, (iii) Fe3O4, (iv) a half metal ferromagnetic material, such as XMnSb (wherein X is at least one selected from Ni, Cu and Pt), LaSrMnO, LaCaSrMnO or CrO2, (v) an oxide, such as a perovskite oxide, a spinel oxide, or a garnet oxide.
In the above-described element, it is also possible that the free magnetic layer includes a first magnetic layer, a non-magnetic layer and a second magnetic layer, layered in that order from the intermediate layer, and when an average film thickness of the first magnetic layer is d1, its average saturation magnetization is M1, an average film thickness of the second magnetic layer is d2, and its average saturation magnetization is M2, then M2xc3x97d2 greater than M1xc3x97d1, and a magneto-resistive element can be devised in which the magnetic resistance displays at least one maximum or minimum in response to a change of the external magnetic field. Here, the two magnetic layers constituting the free magnetic layer are coupled by antiferromagnetic coupling or magnetostatic coupling through the non-magnetic layer, such that it can be made sure that their magnetizations are antiparallel to one another. If the behavior of the magnetization rotation with respect to the size of the magnetic field is different, then it is also possible to use magnetic layers (free magnetic layers) in which magnetization rotations are possible on both sides of the intermediate layer. When two free magnetic layers are arranged so as to sandwich the intermediate layer, at least one free magnetic layer, preferably at least the layer in which magnetization rotated more easily, includes a first magnetic layer, a non-magnetic layer and a second magnetic layer, and the relation M2xc3x97d2 greater than M1xc3x97d1 is satisfied, then the magnetic resistance displays at least one minimum or maximum with respect to changes in the external magnetic field.
When an external magnetic field is applied, first, the second magnetic layer is rotated in the direction of the external magnetic field. The first magnetic layer is coupled to the second magnetic layer by antiferromagnetic or magnetostatic coupling, and is oriented in substantially the opposite direction with respect to the external magnetic field. Furthermore, when a larger external magnetic field is applied, a spin-flop reversal occurs (in the following, magnetostatic coupling reversals with strong coupling are also referred to as xe2x80x9cspin flopsxe2x80x9d), and all magnetic layers are oriented in the direction of the external magnetic field. Consequently, when the magnetization direction of the other magnetic layer flanking the intermediate layer is regarded as substantially constant, the magnetic resistance displays a maximum or minimum with respect to changes of the external magnetic field near the spin flops. Assuming that M2xc3x97d2 less than M1xc3x97d1, this maximum or minimum does not appear clearly. If the two magnetic layers that sandwich the intermediate layer both include a first magnetic layer, a non-magnetic layer, a second magnetic layer, formed in that order from the intermediate layer, and the relation M2xc3x97d2 greater than M1xc3x97d1 is satisfied, wherein an average film thickness of the first magnetic layer is d1, its average saturation magnetization is M1, an average film thickness of the second magnetic layer is d2, and its average saturation magnetization is M2, and if they have a different coercivity or spin flop magnetic field with respect to the external magnetic field, then it is possible to attain at least two maxima or minima.
The maxima or minima with respect to the external magnetic field allow a multi-level response of the magnetic resistance with respect to the external magnetic field. If a magnetic field is applied in a direction in which a spin flop occurs in the free magnetic layer, then it is possible to non-destructively read out, with the change of the magnetic resistance when applying an external magnetic field, the magnetization direction of at least the second magnetic layer that has been as stored the magnetization direction, utilizing the fact that it returns reversibly when the external magnetic field is removed.
The intensity of the spin flop magnetic field can be controlled by the type and thickness of the magnetic films, and the type and thickness of the non-magnetic films. The coercivity can be easily adjusted with the crystal grain size of the material, the crystal magnetic anisotropic energy of the material itself, the element shape, the film thicknesses, and the shape magnetic anisotropic energy, which is a function of the saturation magnetization.
In the above-described element, it is preferable that the free magnetic layer comprises a first magnetic layer, a first non-magnetic layer, a second magnetic layer, a second non-magnetic layer, and a third magnetic layer, layered in that order from the intermediate layer, and when an average film thickness of the magnetic layer n is dn, and its average saturation magnetization is Mn (with n=1, 2, 3), then M3xc3x97d3 greater than M1xc3x97d1 and M3xc3x97d3 greater than M2xc3x97d2; and, with respect to an external magnetic field, a coupling magnetic field of the first magnetic layer and the second magnetic layer may be smaller than a memory reversal magnetic field. In this element, the memory direction of the magnetization of the third magnetic layer is detected with the change of the magnetic resistance when applying a magnetic field that is smaller than the memory reversal magnetic field but larger than the coupling magnetic field in a memory direction of the magnetization of the third magnetic layer.
Here, the third magnetic layer, which is a memory layer, has the highest coercivity or magnetization reversal magnetic field of all magnetic layers constituting the free magnetic layer, and is coupled strongly with the second magnetic layer by antiferromagnetic coupling, ferromagnetic coupling or magnetostatic coupling. The second magnetic layer and the first magnetic layer are coupled by antiferromagnetic coupling or magnetostatic coupling. The other ferromagnetic layer flanking the intermediate layer has a magnetization reversal magnetic field that is high with respect to the free magnetic layer, and it can be regarded substantially as a pinned magnetic layer. If, for example, the third magnetic layer and the second magnetic layer, as well as the second magnetic layer and the first magnetic layer are mutually coupled by antiferromagnetic coupling, then a spin flop occurs between the first magnetic layer and the second magnetic layer due to a step (i) of applying an external magnetic field in a direction that is the same as the magnetization direction stored by the third magnetic layer, and the magnetization of the second magnetic layer changes to parallel to the external magnetic field. In this step (i), there is hardly any change in the magnetization displacement angle between the pinned layer and the first magnetic layer. When removing the external magnetic field, the magnetization of the second magnetic layer returns to its initial state. Of course, it is also possible to use a magnetic field of a strength at which no spin flops occur between the first magnetic layer and the second magnetic layer. On the other hand, in a step (ii) of applying an external magnetic field antiparallel to the magnetization direction that is stored by the third magnetic layer, the magnetization direction of the first magnetic layer is changed to be parallel to the external magnetic field, and the magnetization displacement angle between the pinned magnetic layer and the first magnetic layer changes.
If the external magnetic field is of a strength at which the third magnetic layer is not reverted, then, when the external magnetic field of (ii) is removed, the magnetization of the first magnetic layer returns to its initial state. By applying the external magnetic fields corresponding to these steps (i) and (ii), and detecting the change of the magnetic resistance, it is possible to determine the memory state of the magnetization of the third magnetic layer non-destructively. Usually, in vertical current-type magneto-resistive elements, the element resistance including the wiring resistance is detected, and the magnetization is detected, but because of variations of the element resistance and variations of the wiring resistance, it is not possible to make a decision if there is not a considerable change in the magnetic resistance. In order to overcome these issues, it has been proposed to read the operation voltage of a reference element, but this leads to more complicated wiring and lower circuit integration. There is also the possibility of detecting the memory state by applying an external magnetic field and changing the memory direction, but this destructs the memory state. With the present invention, the memory state of the magneto-resistive element can be detected non-destructively.
In the above-described element, the free magnetic layer is sandwiched by two intermediate layers, and includes magnetic and non-magnetic layers layered in alternation. The two magnetic layers (pinned magnetic layers), in which magnetization rotation is difficult, should be arranged on the outer sides of the two intermediate layers, with respect to the free magnetic layer. In the free magnetic layer sandwiched by the two pinned magnetic layers, the softness of the free magnetic layer and the symmetry of the response with respect to external magnetic fields is significantly harmed by the magnetic coupling with the pinned magnetic layers. If the free magnetic layer is devised as a multilayer structure with magnetostatic coupling or antiferromagnetic coupling, then the influence of the magnetic field leaking from the pinned magnetic layers can be reduced. Moreover, letting the free magnetic layer have a multilayer structure is also effective for suppressing the demagnetizing field of the free magnetic layer that comes with miniaturization.
In the above-described element, it is preferable that the free magnetic layer is sandwiched by two intermediate layers, and is made of 2n magnetic layers (with n being an integer of 1 or greater) and 2nxe2x88x921 non-magnetic layers layered in alternation. Two magnetic layers in which magnetization rotations are difficult (pinned magnetic layers) should be further placed in opposition to the free magnetic layer on the outer sides of the two intermediate layers.
If the coupling among the magnetic layers forming the free magnetic layer is relatively weak, then the magnetization response to external magnetic fields is good. This seems to be because the 2n magnetic layers perform magnetization rotation under the loose constraints of the respective magnetization, so that the demagnetizing energy is suitably lowered. On the other hand, when the magnetic coupling sandwiching the non-magnetic material is strong, there is the effect that the domain structure of the 2n magnetic layers is improved, or the demagnetizing field is suppressed. It should be noted that the strength of the magnetic coupling, such as the magnetostatic coupling or antiferromagnetic coupling between the magnetic layers, can be controlled by the type and thickness of the non-magnetic material.
It is also possible practically to connect two magneto-resistive elements in series. In order to let the two elements connected in series display the largest change of magnetic resistance, the magnetization direction with maximum and minimum of the magnetic resistance of the two elements should be the same. If the polarity of the two magneto-resistive elements (that is, when the magnetic layers sandwiching the intermediate layer are parallel, the resistance is low, when they are antiparallel, the resistance is high, or, when the magnetic layers sandwiching the intermediate layer are parallel, the resistance is high, when they are antiparallel, the resistance is low) is the same, then, when the elements take on the same magnetization arrangement, the largest magnetic resistance changes with respect to changes of the external magnetic field can be attained.
If there are 2n magnetic layers and the magnetic coupling between the magnetic layers constituting the free magnetic layer is sufficiently weaker than the external magnetic field, then any of those 2n layers can be aligned easily with the external magnetic field. In this situation, if the polarity of the two magneto-resistive elements is the same, then the maximum magnetic resistance change can be attained when the magnetization directions of the two pinned magnetic layers are parallel.
If there are 2n magnetic layers and the magnetic coupling between the magnetic layers constituting the free magnetic layer is sufficiently stronger than the external magnetic field, and the magnetization of neighboring magnetic layers is anti-parallel, then, if the polarity of the two magneto-resistive elements is the same, the maximum magnetic resistance change can be attained when the magnetization directions of the two pinned magnetic layers are anti-parallel. The magnetization direction of the pinned magnetic layer can be controlled by changing the number of layers of the ferrimagnetic structure.
In the above-described element, it is preferable that the element includes a first pinned magnetic layer, a first intermediate layer, a first magnetic layer, a non-magnetic layer, a second magnetic layer, a second intermediate layer and a second pinned magnetic layer formed in that order, and when an average film thickness of the magnetic layer n (with n being 1 or 2) is dn, and its average saturation magnetization is Mn, then M2xc3x97d2xe2x89xa0M1xc3x97d1. Here, antiferromagnetic coupling or magnetostatic coupling is performed, so that the magnetization of the first magnetic layer is anti-parallel to the magnetization of the second magnetic layer. By making them anti-parallel, magnetization reversals occur with the difference between M2xc3x97d2 and M1xc3x97d1 serving as an effective magnetic layer.
Taking as an example the case where the polarity of a magneto-resistive element including a first pinned magnetic layer, a first intermediate layer and a first magnetic layer is the same as the polarity of a magneto-resistive element including a second magnetic layer, a second intermediate layer and a second pinned magnetic layer, it is preferable that the magnetization directions of the first pinned layer and the second pinned layer are anti-parallel.
The magnetization directions of the first pinned magnetic layer and the second pinned magnetic layer are made anti-parallel for example when the first pinned magnetic layer is a single ferromagnetic layer in contact with a first antiferromagnetic material, and the second pinned magnetic layer is a configured as ferromagnetic material/non-magnetic material/ferromagnetic material contacting a second antiferromagnetic material. This is only an example, and a similar effect also can be attained when changing the number of layers of the ferrimagnetic structure of the pinned layer, the number of layers of the free magnetic layer, or taking magneto-resistive elements with different polarity, which cases utilize the same principle.
In the above-described elements, it is preferable that at least one free magnetic layer is sandwiched by two intermediate layers, and is made of 2n+1 magnetic layers (with n being an integer of 1 or greater) and 2n non-magnetic layers layered in alternation. Two further magnetic layers in which magnetization rotations are difficult (pinned magnetic layers) should be placed in opposition to the free magnetic layer, on the outer side of the two intermediate layers.
If the coupling among the magnetic layers forming the free magnetic layer is relatively weak, then the magnetization response to external magnetic fields is good. This seems to be because the 2n+1 magnetic layers perform magnetization rotation with loose constraints of the respective magnetization, so that the demagnetizing energy is suitably lowered. On the other hand, when the magnetic coupling sandwiching the non-magnetic material is strong, there is the effect that the domain structure of the 2n+1 magnetic layers is improved, or the demagnetizing field is suppressed. It should be noted that the strength of the magnetic coupling, such as the magnetostatic coupling or antiferromagnetic coupling between the magnetic layers, can be controlled by the type and thickness of the non-magnetic material.
If there are 2n+1 magnetic layers and the magnetic coupling between the magnetic layers constituting the free magnetic layer is sufficiently weaker than the external magnetic field, then any of those 2n+1 layers can be aligned easily with the external magnetic field. In this situation, if the polarity of the two magneto-resistive elements is the same, then the maximum magnetic resistance change can be attained when the magnetization directions of the two pinned magnetic layers are parallel.
Similarly, if there are 2n+1 magnetic layers and the magnetic coupling between the magnetic layers constituting the free magnetic layer is sufficiently stronger than the external magnetic field, and the magnetization of neighboring magnetic layers is anti-parallel, then, if the polarity of the two magneto-resistive elements is the same, the maximum magnetic resistance change can be attained when the magnetization directions of the two pinned magnetic layers are parallel.
It is preferable that the magneto-resistive element includes a first pinned magnetic layer, a first intermediate layer, a first magnetic layer, a first non-magnetic layer, a second magnetic layer, a second non-magnetic layer, a third magnetic layer, a second intermediate layer and a second pinned magnetic layer formed in that order, and that, when an average film thickness of the magnetic layer n is dn (with n being 1, 2 or 3), and its average saturation magnetization is Mn, then M3xc3x97d3+M1xc3x97d1xe2x89xa0M2xc3x97d2.
Here, the first magnetic layer, the second magnetic layer and the third magnetic layer are coupled by antiferromagnetic coupling or magnetostatic coupling, so that their magnetizations are anti-parallel to one another. When the relation M3xc3x97d3+M1xc3x97d1 greater than M2xc3x97d2 is satisfied, the first magnetic layer and the third magnetic layer function effectively as magnetic layers with respect to external magnetic fields. In that case, because the second magnetic layer is magnetically coupled to the first magnetic layer and the third magnetic layer, it does not function apparently as a magnetic layer with respect to external magnetic fields, but when the first magnetic layer and the third magnetic layer perform a magnetization rotation in response to the external magnetic field, it performs a substantially simultaneous magnetization rotation while maintaining antiferromagnetic coupling or magnetostatic coupling with those layers. On the other hand, when the relation M3xc3x97d3+M1xc3x97d1 less than M2xc3x97d2 is satisfied, then the second magnetic layer functions effectively as a magnetic layer with respect to external magnetic fields. In that case, because the first magnetic layer and the third magnetic layer are magnetically coupled to the second magnetic layer, they do not function apparently as magnetic layers, but when the second magnetic layer perform a magnetization rotation in response to the external magnetic field, they performs a substantially simultaneous magnetization rotation.
If the polarity of a magneto-resistive element including a first pinned magnetic layer, a first intermediate layer and a first magnetic layer is the same as the polarity of a magneto-resistive element including a third magnetic layer, a second intermediate layer and a second pinned magnetic layer, then the maximum magnetic resistance change can be attained when the magnetization directions of the first pinned magnetic layer and the second pinned magnetic layer are the same. However, a similar effect also can be attained when changing the number of layers of the ferrimagnetic structure of the pinned magnetic layer, the number of layers of the free magnetic layer, or taking magneto-resistive elements with different polarity, which cases utilize the same principle.
In the above-described elements, it is preferable that at least one of the magnetic layers of the free magnetic layer has a coercivity or saturation magnetization that is different from the other magnetic layers.
For example, consider the case that the free magnetic layer is a multilayer film of magnetic and non-magnetic layers, and when taking the magnetic layers arranged at positions m (m=1, 2, . . . ) from the intermediate layer as magnetic layers m, and taking the product Mmxc3x97dm of average saturation magnetization Mm of the magnetic layers m and their average film thickness dm, the sum of the products Mmxc3x97dm for odd m is substantially the same as the sum of the products Mmxc3x97dm for even m, and the magnetic layers are coupled by antiferromagnetic coupling or magnetostatic coupling. In that configuration, if during the microprocessing, all magnetic layers have the same coercivity by appropriate selection of materials and film thicknesses, then the magnetization rotation of the free magnetic layer is easy, whereas the storage of the magnetization state is difficult. However, when using for one magnetic layer a material whose coercivity is higher than that of the other magnetic layers, the demagnetizing field effect occurring due to the miniaturization can be suppressed, and a suitable coercivity for storing the magnetization state in the free magnetic layer can be created. This configuration has the advantage that a more stable and suitable coercivity can be attained than with storage by uniaxial anisotropy utilizing shape anisotropy, which depends strongly on the precision of the element miniaturization, and in which an overly large coercivity results easily from the miniaturization.
As another example, consider the case that, when taking the magnetic layers constituting the free magnetic layers arranged at positions m (m=1, 2, . . . ) from the intermediate layer as magnetic layers m, and taking the product Mmxc3x97dm of average saturation magnetization Mm of the magnetic layers m and their average layer thickness dm, the sum of the products Mmxc3x97dm for odd m is different from the sum of the products Mmxc3x97dm for even m, and the magnetic layers are coupled by antiferromagnetic coupling or strong magnetostatic coupling. The reversal magnetic field of the miniaturized elements is substantially proportional to the difference of the product Mmxc3x97dm of the odd layers and the even layers. Mm or dm can be controlled to preserve a suitable reversal magnetic field in the microprocessed elements. However, in microprocessed elements of submicron dimension, there are technical limitations to the actual control of the film thickness, and variations in the reversal magnetic fields occur. Consequently, the variations in the reversal magnetic fields of elements in large areas can be controlled when a material with small M is chosen for the magnetic layers responsible for the reversal magnetic field.
It also is preferable to combine at least two magneto-resistive elements. A magneto-resistive element A has a free magnetic layer as described above, wherein the free magnetic layer comprises a first magnetic layer, a first non-magnetic layer and a second magnetic layer, layered in that order from the intermediate layer, and wherein M2xc3x97d2 greater than M1xc3x97d1. A magneto-resistive element B has an intermediate layer and a free magnetic layer, wherein the free magnetic layer comprises a first magnetic layer and a second magnetic layer, positioned in that order from the intermediate layer, and wherein M3xc3x97d3 greater than M4xc3x97d4. The element A and the element B respond to the same external magnetic field, and the output of element A and B is added to or subtracted from one another. The free magnetic layer in the element B may be composed of magnetic layers.
In this configuration, the first magnetic layer of the element A and the second magnetic layer of the element B point in the same direction as the external magnetic field. Consequently, the first magnetic layer in opposition to the intermediate layer points into different directions in element A and element B. If the pinned layers in element A and element B are oriented in the same direction, then, with respect to external magnetic fields of the same direction, the orientation of the external magnetic field at which element A has the largest resistance is different from that at which element B has the largest resistance. When used as a device, the peripheral circuit resistance is added to the resistance value of the magneto-resistive element, so that an adequate S/N ratio cannot be attained, but when combining the elements A and B as described above, the base resistance (circuit resistance+element resistance for low resistances) is canceled, thus attaining a high S/N ratio.
In another configuration of the present invention, the vertical current-type magneto-resistive element includes an intermediate layer; and a pair of magnetic layers sandwiching the intermediate layer; wherein one of the magnetic layers is a pinned magnetic layer in which magnetization rotation with respect to an external magnetic field is more difficult than in the other magnetic layer; wherein the pinned magnetic layer is in contact with a primer layer or an antiferromagnetic layer; and wherein an element area, which is defined by the area of the intermediate layer through which current flows perpendicular to the film plane, is not larger than 1000 xcexcm2, preferably not larger than 10 xcexcm2, more preferably not larger than 1 xcexcm2, most preferably not larger than 0.1 xcexcm2.
The pinned magnetic layer has a multilayer structure including a non-magnetic layer and magnetic layers sandwiching the non-magnetic layer, and a thickness d of the non-magnetic layer is in the range of 0.3 nm less than d less than 2.6 nm.
It is preferable that magnetization rotation of the pinned magnetic layer occurs at field strengths that are at least 50 Oe higher than for the other magnetic layer (free magnetic layer).
With this configuration, the non-magnetic layer and the magnetic layers are coupled by antiferromagnetic coupling, the domains are simplified, and a high MR can be attained. In this situation, when d is 0.3 nm or less, the thermal stability deteriorates. When d is 2.6 nm or higher, the afore-mentioned magnetostatic coupling becomes dominant. If thermal processing is performed at temperatures of 300xc2x0 C. or higher, then it is preferable that d is in the range of 0.7 nm to 2.6 nm. It is preferable that the thickness of the magnetic layers is about 0.3 nm to 10 nm, in which range strong antiferromagnetic coupling is attained.
For the non-magnetic material, it is possible to use a conductive metal or a metal compound, in particular Cu, Ag, Au, Ru, Rh, Ir, Re or Os. Furthermore, it is also possible to use metal alloys of these, or an alloy or compound containing at least 50 wt % of these metals.
Magnetization rotation with respect to external magnetic fields can be made difficult with a configuration in which, when the magnetic layers m are the magnetic layers in the pinned magnetic layer that are arranged at positions m (with m being an integer of 1 or greater) from the intermediate layer, Mm is an average saturation magnetization of the magnetic layers m and dm is their respective average layer thickness, then the sum of the products Mmxc3x97dm for odd m is substantially equal to the sum of the products Mmxc3x97dm for even m.
However, when the free magnetic layer is domain controlled by a bias magnetic field, or when the free magnetic layer is positively coupled with the intermediate layer, due to the so-called orange peel effect or the like, then it is also possible to generate a bias by letting the sum of the products Mmxc3x97dm for odd m slightly deviate from the sum of the products Mmxc3x97dm for even m, in order to rectify magnetic field shifts. However, it is preferable that the upper limit for this slight deviation is not larger than 2 nmT, expressed by saturation magnetizationxc3x97film thickness.
It is also possible to combine two or more elements, including a magneto-resistive element A including a pinned magnetic layer, in which 2n imagnetic layers and 2nxe2x88x921 non-magnetic layers (with n being an integer of 1 or greater) are layered in alternation from the intermediate layer; and a magneto-resistive element B, in which 2n+1 magnetic layers and 2n non-magnetic layers are layered in alternation from the intermediate layer (with n=1, 2, 3 . . . ); wherein the element A and the element B respond to the same external magnetic field; and wherein the outputs of element A and element B are added to or subtracted from one another. Here, it is preferable that the magnetization directions of the magnetic layers constituting the pinned magnetic layers are antiparallel, due to antiferromagnetic coupling or magnetostatic coupling.
With this configuration, if the polarity of the element A and the element B is the same (that is, if the magnetization directions of the magnetic layers sandwiching the intermediate layer are parallel, then the resistance is low (or high), and if the magnetization directions are antiparallel, then the resistance is high (or low)), then, with respect to external magnetic fields from the same direction, the directions of the external magnetic fields for which element A has its maximum resistance and for which B has its maximum resistance are different. When used as a device, the peripheral circuit resistance is added to the resistance value of the magneto-resistive element, so that an adequate S/N ratio cannot be attained, but when combining the elements A and B as described above, the base resistance (circuit resistance +element resistance for low resistances) is canceled, thus attaining a high S/N ratio.
The thermal resistance is further improved when at least a portion of the non-magnetic material of the free magnetic layers, or of the non-magnetic material of the pinned magnetic layers is made of at least one compound selected from oxides, nitrides, carbides and borides. This is, because the energy value for these compounds is more stable than the energy value for mutual diffusions with the magnetic layers. However, overall, the non-magnetic layers should have a sufficiently low resistance.
As an example in which a portion of the non-magnetic material is of the above-mentioned compound, the non-magnetic film can be a multilayer film of at least one layer of non-magnetic material including at least one selected from oxides, nitrides, carbides and borides, and at least one non-magnetic metal layer. For example, the thermal resistance improves when using a multilayer film of at least two layers including a non-magnetic metal X (Xxe2x95x90Cu, Ag, Au, Ru, Rh, Ir, Re, Os) and an R selected from oxides, nitrides, carbides and borides (Rxe2x95x90SiO2, SiC, Si3N4, Al2O3, AlN, Cr2O3, Cr2N, TiO, TiN, TiC, HfO2, HfN, HfC, Ta2O5, TaN, TaC, BN, B4C. or mixtures of the above).
When using a primer layer that includes at least one element selected from the elements of groups IVa to VIa and VIII (but excluding Fe, Co and Ni) and Cu, then, especially when the pinned layer is a multilayer film including magnetic and non-magnetic layers, strong antiferromagnetic coupling is attained directly after the film formation, without using an antiferromagnetic material such as IrMn, FeMn or the like. Moreover, since no Mn-including antiferromagnetic material is used, the decrease of the MR due to Mn diffusion during the thermal process can be suppressed. The primer layer preferably is composed of the above at least one element.
If the primer layer is in contact with a magnetic layer, and the primer layer and that magnetic layer include at least one crystal structure selected from fcc and hcp structure, or both include a bcc structure, then the antiferromagnetic coupling between the magnetic layers of the pinned layer becomes particularly strong.
If the antiferromagnetic layer is made of Cr and at least one selected from the group consisting of Mn, Tc, Ru, Rh, Re, Os, Ir, Pd, Pt, Ag, Au and Al, then an excellent thermal resistance is attained. This alloy also can include up to 10 at % of an element other than those mentioned above.
A particularly superior thermal resistance can be attained if the antiferromagnetic layer has a composition that can be expressed by Cr100-xMex (wherein Me is at least one selected from the group consisting of Re, Ru and Rh, and 0.1xe2x89xa6Xxe2x89xa620).
An excellent thermal resistance can be attained in particular when the antiferromagnetic layer has a composition that can be expressed by Mn100-xMex (wherein Me is at least one selected from the group consisting of Pd and Pt, and 40xe2x89xa6Xxe2x89xa655). This seems to be because a higher amount of noble metals is contained than in IrMn or FeMn. It is also possible that the MnMe composition contains up to 10 at % of another element such as Tc, Ru, Rh, Re, Os, Ir, Pd, Pt, Ag, Au or Al.
The thermal resistance is further improved when the antiferromagnetic layer is formed on a primer layer, and the primer layer and the antiferromagnetic layer include at least one crystal structure selected from fcc, fct, hcp and hct structure, or the primer layer and the antiferromagnetic layer both include a bcc structure. This seems to be because the crystal growth of the antiferromagnetic layer is promoted by the primer layer, and by reducing strain, the diffusion of elements constituting the antiferromagnetic material can be suppressed.
The crystallinity of the antiferromagnetic layer can be improved in particular when the antiferromagnetic layer can be expressed by Mn100-xMex (wherein Me is at least one selected from the group consisting of Pd and Pt, and 40xe2x89xa6Xxe2x89xa655), and the primer layer is made of NiFe or NiFeCr. In that case, a considerable increase of thermal resistance for thermal processing at temperatures of at least 300xc2x0 C. can be observed.
This seems to be the effect of decreasing strain by increasing the crystallinity, and suppressing grain growth during the thermal processing.
When the magnetic layer in contact with the antiferromagnetic layer is made of Co, the mutual diffusion of antiferromagnetic material is suppressed even better.
It is preferable that at least a portion of the magnetic layer in contact with at least one selected from the antiferromagnetic layer and the non-magnetic layer is made of a ferromagnetic material including at least one element selected from the group consisting of oxygen, nitrogen and carbon. This is because the deterioration of the MR based on inter-layer diffusion during the thermal process can be suppressed by using a material that is relatively thermally stable. This magnetic layer also can be of (i) a three-layer structure such as (metal ferromagnetic material)/(ferromagnetic material including oxygen, nitrogen or carbon)/(metal ferromagnetic material), (ii) a two-layer structure such as (ferromagnetic material including oxygen, nitrogen or carbon)/(metal ferromagnetic material), or (iii) the entire magnetic layer can be made of a ferromagnetic material including oxygen or nitrogen. As a ferromagnetic material including oxygen, it is preferable to use spinel oxides for which ferrite materials such as Fe3O4, MnZn ferrite and NiZn ferrite are typical examples, garnet oxides, perovskite oxides, or oxide ferromagnetic materials such as TMO (with T being one selected from Fe, Co and Ni, and M being one selected from Al, Si, Ti, Zr, Hf, V, Nb, Ta, Cr and Mg). As a ferromagnetic material including nitrogen, it is preferable to use a ferromagnetic nitride material such as TN or TMN (with T and M as above). As a ferromagnetic material including carbon, it is preferable to use a ferromagnetic carbide material such as TMC (with T and M as above).
In the above-described elements, it is also possible that at least a portion of the magnetic layer in contact with at least one selected from the antiferromagnetic layer and the non-magnetic layer is made of an amorphous ferromagnetic material, because this suppresses inter-layer diffusion during thermal processing. As an amorphous material, it is preferable to use Co(Fe)NbZr, Co(Fe)TaZr, CoFeB, Co(Fe)Ti, Co(Fe)Zr, Co(Fe)Nb, Co(Fe)MoBZr, Co(Fe)VZr, Co(Fe)MoSiZr, Co(Fe)MoZr, Co(Fe)MoVZr or Co(Fe)MnB.
It is preferable that, when df is a thickness of the pinned magnetic layer and da is a thickness of the ferromagnetic layer, then
2 nmxe2x89xa6dfxe2x89xa650 nm,
5 nmxe2x89xa6daxe2x89xa6100 nm,
0.1xe2x89xa6df/daxe2x89xa65.
If the pinned magnetic layer is thinner than 2 nm, then a magnetic deterioration of the pinned layer due to diffusion of antiferromagnetic material may occur. If the pinned magnetic layer is thicker than 50 nm, then domain disturbances may occur due to the increase of the antiferromagnetic field in the film plane caused by miniaturization, or it may not be possible to attain a sufficient pinning magnetic field due to the antiferromagnetic material. If the antiferromagnetic material is thinner than 5 nm, then it may not be possible to attain a sufficient pinning magnetic field. If the antiferromagnetic material is thicker than 100 nm, then inter-layer diffusion may promote magnetic field deterioration of the pinning layer. Especially after thermal processing of at least 350xc2x0 C., a preferable ratio of df and da with little deterioration of the MR is 0.1xe2x89xa6df/daxe2x89xa65. If df/da is smaller than 0.1, then inter-layer diffusion tends to occur, and if it is larger than 5, then the pinning magnetic field due to the antiferromagnetic material tends to become weak, or domain disturbances due to the demagnetizing field tend to become large.
It is also possible that the above-described element is formed on a lower electrode made of a metal multilayer film. The lower electrode through which current flows into the element should have a thickness of several dozen nm to several hundred nm, in order to suppress property variations due to shape effects. Therefore, depending on the thermal processing temperature, grain growth cannot be ignored and contributes to property variations. It is possible to increase the thermal resistance by providing the lower electrode with a multilayer structure of different materials, in order to suppress grain growth.
It is preferable that the metal multilayer film is a multilayer film including a highly conductive metal layer having at least one selected from the group consisting of Ag, Au, Al and Cu as a main component, and a grain-growth suppression layer of a metal (i) having at least one element selected from groups IVa to VIa and VIII (Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W. Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt) as a main component, or (ii) of a compound selected from the group consisting of conductive oxides, conductive nitrides and conductive carbides. It is preferable that the thickness of the highly conductive metal film is about several nm to 100 nm and that the thickness of the grain-growth suppression layer is about several nm to several nm. In this specification, a main component means a component of 50 wt % or more in the composition.
If the intermediate layer is made of an insulator or a semiconductor including at least one element selected from the group consisting of oxygen, nitrogen, carbon and boron, then a vertical current-type magneto-resistive element utilizing the tunneling magnetic resistance effect can be attained. Examples of preferable materials for the intermediate layer include SiO2, SiC, Si3, N4, Al2O3, AlN, Cr2O3, TiC, HfO2, HfN, HfC, Ta2O5, TaN, TaC, BN, B4C., DLC(Diamond like Carbon) and C60, as well as mixtures of these compounds.
If the intermediate layer is made of at least one metal selected from transition metals, or at least one conductive compound selected from compounds of transition metals with oxygen, nitrogen and carbon, then a vertical current-type magneto-resistive element utilizing the CPP-GMR effect having lower coercivity and high thermal resistance can be produced. For this CPP-GMR element, it is preferable that the element area is not larger than 0.01 xcexcm2.
When the element area is not larger than 0.01 xcexcm2, then the element resistance can be improved, and the problems of deterioration of coercivity due to the miniaturization and deterioration of the thermal resistance both can be solved.
For the intermediate layer, it is preferable to use at least one of the transition metals, in particular V, Nb, Ta, Cr, Mo, W, Cu, Ag, Au, Ru, Rh, Ir, Re or Os. The element resistance can be improved and the thermal resistance can be improved by using a conductive compound of these elements that has been oxidized, nitrided or carbided to an extent at which their conductivity is not lost; or an oxide, nitride, carbide or boride compound XR of an R (with Rxe2x95x90SiO2, SiC, Si3N4, Al2O3, AlN, Cr2O3, Cr2N, TiO, TiN, TiC, HfO2, HfN, HfC, Ta2O5, TaN, TaC, BN, B4C. or composites of the above) with the above-mentioned transition metals X; or a multilayer film X/R of at least two layers.
If at least a portion of at least one of the magnetic layers sandwiching the intermediate layer comprises a ferromagnetic material including oxygen, nitrogen or carbon, or an amorphous ferromagnetic material, then it is possible to increase the element resistance of the vertical current-type magneto-resistive element due to the CPP-GMR effect. As a ferromagnetic material including oxygen, it is preferable to use spinel oxides for which ferrite materials such as Fe3O4, MnZn ferrite and NiZn ferrite are typical examples, garnet oxides, perovskite oxides, or oxide ferromagnetic materials such as TMO (with T being one selected from Fe, Co and Ni, and M being one selected from Al, Si, Ti, Zr, Hf. V, Nb, Ta, Cr and Mg). As a ferromagnetic material including nitrogen, it is preferable to use a ferromagnetic nitride material such as TN or TMN (with T and M as above). As a ferromagnetic material including carbon, it is preferable to use a ferromagnetic carbide material such as TMC (with T and M as above). As an amorphous material, it is preferable to use, for example, Co(Fe)NbZr, Co(Fe)TaZr, CoFeB, Co(Fe)Ti, Co(Fe)Zr, Co(Fe)Nb, Co(Fe)MoBZr, Co(Fe)VZr, Co(Fe)MoSiZr, Co(Fe)MoZr, Co(Fe)MoVZr or Co(Fe)MnB.
If the free magnetic layer of the vertical current-type magneto-resistive element serves as a magnetic memory layer, then it can be used as a memory element with high SIN ratio and low power consumption.
If at least a portion of the free magnetic layer of the vertical current-type magneto-resistive element serves as a flux guide, then the element can be used as a magnetic reproduction element with high SIN ratio and low Barkhausen noise.
When xe2x80x9caxe2x80x9d is the longest width of the element shape of the free magnetic layer, xe2x80x9cbxe2x80x9d is its shortest width, and a/b is in the range of 5 less than a/b less than 10, then a magneto-resistive element with high memory stability or a magneto-resistive element with high reproduction sensitivity can be produced.
If the vertical current-type magneto-resistive element is subjected first to heat treatment at 300xc2x0 C. to 450xc2x0 C., and then to heat treatment in a magnetic field at 200xc2x0 C. to 400xc2x0 C., then excellent MR properties can be attained. Here, the former heat treatment may be thermal processing that is e.g. sintering in hydrogen containing atmosphere or formation of a passivation film. Performing heat treatment in a magnetic field after thermal processing near the Neel point or the blocking temperature, the pinned magnetization direction of the antiferromagnetic material in particular is set uniformly.
Especially with elements using an antiferromagnetic layer, an even stronger pinned magnetic field can be attained by heat treatment in a magnetic field at 300xc2x0 C. to 450xc2x0 C.
If the vertical current-type magneto-resistive element of the present invention is mounted together with other semiconductor devices, then a heat treatment at 200xc2x0 C. to 350xc2x0 C. should be performed in a magnetic field after forming a multilayer film made of at least one antiferromagnetic layer, a pinned magnetic layer, an intermediate layer and a free magnetic layer on a substrate provided with, for example, CMOS semiconductor elements and lead electrodes, and the antiferromagnetic material and free magnetic material etc. are subjected to a uniaxial anisotropy formation step.
It is preferable that this step is performed before the miniaturization of the magnetic multilayer film, when the influence of the demagnetizing film is smallest. In the process of element microprocessing and electrode wiring, oxides of the electrodes etc. are reduced, and with the goal of reducing the wiring resistances, heat treatment is performed in a reducing atmosphere, such as a hydrogen-containing atmosphere, of 300xc2x0 C. to 450xc2x0 C. To perform further heat treatment in a magnetic field after performing the heat treatment in the reducing atmosphere is advantageous with regard to the element properties, and considering the oxidation of the electrodes etc., it is advantageous with regard to the manufacturing process to perform the heat treatment in the magnetic field beforehand. In particular, PtMn and PtPdMn are preferable as antiferromagnetic materials.
It is also possible to devise a portable device equipped with a plurality of vertical current-type magneto-resistive elements, wherein data that have been communicated by electromagnetic waves are stored in the free magnetic layers of the vertical current-type magneto-resistive elements. With such a device, it is possible to realize low power consumption due to the low coercivity, in addition to the non-volatility and speed of MRAMs, so that it can be used as a memory necessary for high-speed reading and writing of large capacities, as for video or audio.
The magneto-resistive element can also include a first pinned magnetic layer, a first intermediate layer, a first free magnetic layer, a non-magnetic conductive layer, a second free magnetic layer, a second intermediate layer and a second pinned magnetic layer formed in that order, wherein at least one of the first free magnetic layer and the second free magnetic layer includes one or more magnetic layers and one or more non-magnetic layers layered in alternation.
By electrically connecting, through a non-magnetic conductive layer, in series a first pinned magnetic layer, a first intermediate layer, a first free magnetic layer, a second free magnetic layer, a second intermediate layer and a second pinned magnetic layer, the deterioration of the magnetic resistance at identical bias can be diminished, if the intermediate layers are insulating layers using the tunneling effect. Moreover, if the intermediate layers are made of a conductive material utilizing the CPP-GMR effect, the element resistance can be increased by the serial connection. If the free magnetic layers are in the region sandwiched by the two intermediate layers, variations of the magnetization reversals due to the distribution of the external magnetic field can be suppressed. The demagnetizing field due to miniaturization can be diminished, when at least one of the free magnetic layers has magnetic and non-magnetic layers layered in alternation.
In the magneto-resistive element, it is also preferable that magnetic layers that are adjacent but spaced apart by a non-magnetic conductive layer are magnetized antiparallel to one another. If magnetic layers that are adjacent but spaced apart by a non-magnetic conductive layer are magnetized parallel to one another, then the magnetostatic energy of the neighboring magnetization through the non-magnetic conductive layer is increased, so that non-symmetries with respect to the external magnetic fields occur. Therefore, even when the two free magnetic layers are on the same side of the intermediate layer, the responsiveness of magnetic resistance changes with respect to the external magnetic field deteriorates as a result. By making them antiparallel, the increase of the magnetostatic energy can be minimized, and the element driving stability during miniaturization can be improved.
In the magneto-resistive element, it is preferable that the non-magnetic conductive layer has a thickness of 2.6 nm to 50 nm. If the non-magnetic conductive layer has a thickness of less than 2.6 nm, then antiferromagnetic exchange coupling or ferromagnetic exchange coupling becomes stronger, which is undesirable. If it has a thickness of more than 50 nm, then the influence of the distribution of the external magnetic field cannot be ignored, and the size of the magnetic field felt by the two free magnetic layers may be different. There is no particular limitation with regard to the material of the non-magnetic conductive layer, and it is preferable to use non-magnetic materials that are commonly used for conductive electrode materials, such as Cu, Al, TiN, TiWN, CuAl, CuAlTi, Ag, Au or Pt, whose specific resistance is 11000 xcexa9cm or less.
It is preferable that the magneto-resistive element includes four pinned magnetic layers, two free magnetic layers, and four intermediate layers, wherein at least one of the free magnetic layers is made of one or more magnetic layers and one or more non-magnetic layers layered in alternation.
A representative example of the above configuration is first antiferromagnetic layer/first pinned magnetic layer/first intermediate layer/first free magnetic layer/second intermediate layer/second pinned magnetic layer/second antiferromagnetic layer/third pinned magnetic layer/third intermediate layer/second free magnetic layer/fourth intermediate layer/fourth pinned magnetic layer/third antiferromagnetic layer. If the pinned magnetic layers are made of a magnetic material with high coercivity, or if they are made of layered ferrimagnetic material, then the antiferromagnetic layers are not always necessary. With this configuration, the total thickness of a TMR element having four intermediate layers or a CPP-GMR element can be minimized, so that it becomes possible to make smaller devices having four intermediate layers. Furthermore, for TMR elements, the bias dependency can be improved considerably, and in CPP-GMR elements, the resistance can be increased. Deteriorating properties due to the demagnetizing field can be suppressed by making the free magnetic layer a multilayer including magnetic and non-magnetic layers. By making the pinned layer a multilayer including magnetic and non-magnetic layers, the magnetic non-symmetry of the free magnetic layers can be improved.
It is also preferable that the magneto-resistive element includes a pinned magnetic layer, an intermediate layer and a free magnetic layer, wherein the free magnetic layer is in contact with a buffer layer, wherein the buffer layer is made of a composition that includes 10 wt % to 50 wt % of a non-magnetic element in a magnetic composition in contact with the buffer layer, and wherein the saturation magnetization of that composition is not more than 0.2 T. If the free magnetic layer is made thinner than 2 nm in order to suppress the demagnetizing field, then the magnetic properties deteriorate because of disturbances of the crystal structure due to making w the layer thin and interface reactions during production. By using, as a buffer layer in contact with the free magnetic layer, a material in which a non-magnetic element has been added to the magnetic material composition constituting the free magnetic layer, if the buffer layer is used as a primer of the free magnetic layer for example, then the crystallinity of the free magnetic layer improves, and if it is used above the free magnetic layer, then it has the effect that the deterioration of the magnetic field due to interface reactions can be suppressed. The free magnetic layer can be a single layer, or it can be a multilayer including magnetic and non-magnetic layers. In the latter case, it is preferable that the buffer layer contacts the magnetic layer.
It is preferable that the saturation magnetization of the buffer layer is not greater than 0.2 T. With regard to maintaining the crystal structure of the buffer layer and suppressing the saturation magnetization, it is also preferable that the non-magnetic element added to the composition of the magnetic layer accounts for 10 wt % to 50 wt %.
It is also preferable that the buffer layer comprises at least one non-magnetic element selected from the group consisting of Cr, Mo and W. By forming an alloy with, for example, Fe, Ni, Co, FeNi, FeCo or CoFeNi, these elements are very effective for making the free magnetic layer thin. Of these materials, an alloy NiFeCr with the magnetic material NiFe is preferable, in particular near the composition (NiFe)61Cr39.
It is also preferable that the free magnetic layer is made of at least one non-magnetic layer and magnetic layers sandwiching the non-magnetic layer, and that a total film thickness of the magnetic layers is at least 4 nm. By making the free magnetic layer a multilayer including magnetic and non-magnetic layers, it is possible to suppress an increase in the coercivity that is brought about by making the element smaller. If the free magnetic layer is used as a memory device, there is the advantage that the power consumption drops when memory reversals occur using a current-generating magnetic field. However, on the other hand, with the thermal stability index as expressed by KuV/kBT (wherein Ku is the uniaxial anisotropic energy, V is the volume, and kBT is the Boltzman constant and absolute temperature), when Ku (coercivity=2 Ku/M, wherein M is magnetization) is made small, the thermal stability of the storage becomes poor. However, by increasing the volume V of the magnetic layer included in the multilayered memory (free magnetic layer), the thermal stability can be improved, and it is preferable that the total thickness of the magnetic layers is at least 4 nm, more preferably at least 8 nm.
In the most simple basic configuration of a magneto-resistive element in accordance with the present invention, a current flows through an intermediate layer that is sandwiched by at least two magnetic layers. Most simply, a layering structure of lower electrode/(primer layer or antiferromagnetic layer)/pinned magnetic layer/intermediate layer/free magnetic layer is formed by gas-phase film formation on an insulating or conducting substrate. Alternatively, a layering structure of lower electrode/(primer layer)/free magnetic layer/intermediate layer/pinned magnetic layer/(antiferromagnetic layer) is formed. This multilayer film is processed into a mesa shape, and after the side walls are covered with an inter-layer insulator, an upper electrode is formed. With current flowing between the lower electrode and the upper electrode, a voltage change is read corresponding to the change of the magnetization displacement angle depending on the magnetization between the magnetic layers.
For the gas-phase film formation for forming the magnetic layers, antiferromagnetic layer, inter-layer insulating layer, electrodes, intermediate layer, etc., it is possible to use any of the PVD methods ordinarily used for thin film formation, such as ion beam deposition (IBD), cluster ion beam deposition, or sputtering methods, such as RF, DC, ECR, helicon, ICP sputtering or sputtering with opposing targets, MBE, or ion plating, or any other suitable method. In particular for making the interlayer insulating film and the intermediate layer, it is also possible to use CVD. Furthermore, if the layers are made of an oxide, nitride, carbide or boride, they also can be produced by chemical beam epitaxy, gas source MBE, reactive vapor deposition, or reactive sputtering, or it is possible to form them after the gas-phase film formation by reacting atoms, molecules, ions (plasma), radicals or the like, controlling partial pressures, reaction temperature and time as suitable. Plasma or radicals can be generated by ECR discharge, glow discharge, RF discharge, helicon discharge or ICP discharge.
The mesa processing of the magneto-resistive element of the present invention can be performed by any process that is ordinarily used for microprocessing, for example physical or chemical etching, such as ion milling, RIE, EB, or FIB etching, or photolithography techniques using suitable line widths. In order to make the lower electrode flat, it is also effective to use CMP or cluster ion beam etching to increase the MR.